Network Working Group Seisho Yasukawa (NTT)
Internet Draft Editor
Category: Informational
Expiration Date: May 2005 December 2004
Signaling Requirements for Point to MultipointTraffic Engineered MPLS LSPs<draft-ietf-mpls-p2mp-sig-requirement-00.txt>
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Abstract
This document presents a set of requirements for the establishment
and maintenance of Point-to-Multipoint (P2MP) Traffic Engineered (TE)
Multiprotocol Label Switching (MPLS) Label Switched Paths (LSPs).
There is no intent to specify solution specific details nor
application specific requirements in this document.
It is intended that the requirements presented in this document are
not limited to the requirements of packet switched networks, but also
encompass the requirements of L2SC, TDM, lambda and port switching
networks managed by Generalized MPLS (GMPLS) protocols. Protocol
solutions developed to meet the requirements set out in this document
must attempt to be equally applicable to MPLS and GMPLS.
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Internet Draft draft-ietf-mpls-p2mp-sig-requirement-00.txt December 20041. Introduction
Existing MPLS Traffic Engineering (MPLS-TE) allows for strict QoS
guarantees, resources optimization, and fast failure recovery, but is
limited to point-to-point (P2P) applications. Requirements have been
expressed for the provision of services over point-to-multipoint
(P2MP) traffic engineered tunnels and this clearly motivates
enhancements of the base MPLS-TE tool box in order to support P2MP
MPLS-TE LSPs.
[RFC2702] specifies requirements for traffic engineering over MPLS.
It describes traffic engineering in some detail, and those
definitions and objectives are equally applicable to traffic
engineering in a point-to-multipoint service environment. They are
not repeated here, but it is assumed that the reader is fully
familiar with them.
[RFC2702] also explains how MPLS is particularly suited to traffic
engineering, and presents the following eight reason.
1. Explicit label switched paths which are not constrained by
the destination based forwarding paradigm can be easily created
through manual administrative action or through automated
action by the underlying protocols.
2. LSPs can potentially be efficiently maintained.
3. Traffic trunks can be instantiated and mapped onto LSPs.
4. A set of attributes can be associated with traffic trunks
which modulate their behavioral characteristics.
5. A set of attributes can be associated with resources which
constrain the placement of LSPs and traffic trunks across
them.
6. MPLS allows for both traffic aggregation and disaggregation
whereas classical destination only based IP forwarding
permits only aggregation.
7. It is relatively easy to integrate a "constraint-based routing"
framework with MPLS.
8. A good implementation of MPLS can offer significantly lower
overhead than competing alternatives for Traffic Engineering.
These points are equally applicable to point-to-multipoint
traffic engineering. Points 1. and 7. are particularly important.
That is, the traffic flow for a point-to-multipoint LSP is not
constrained to the path or paths that it would follow during
multicast routing or shortest path destination-based routing, but
can be explicitly controlled through manual or automated action.
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Further, the explicit paths that are used may be computed using
algorithms based on a variety of constraints to produce all manner of
tree shapes. For example, an explicit path may be cost-based
[STEINER], shortest path, QoS-based, or may use some fair-cost QoS
algorithm.
[RFC2702] also describes the functional capabilities required to
fully support Traffic Engineering over MPLS in large networks.
1. A set of attributes associated with traffic trunks which
collectively specify their behavioral characteristics.
2. A set of attributes associated with resources which constrain
the placement of traffic trunks through them. These can also be
viewed as topology attribute constraints.
3. A "constraint-based routing" framework which is used to select
paths for traffic trunks subject to constraints imposed by
items 1) and 2) above. The constraint-based routing framework
does not have to be part of MPLS. However, the two need to be
tightly integrated together.
These basic requirements should also be supported by P2MP traffic
engineering.
This document presents a set of requirements for
Point-to-Multipoint(P2MP) Traffic Engineering (TE) extensions to
Multiprotocol Label Switching (MPLS). It specifies functional
requirements for solutions to deliver P2MP TE LSPs.
It is intended that solutions that specify procedures for P2MP TE LSP
setup satisfy these requirements. There is no intent to specify
solution specific details nor application specific requirements in
this document.
It is intended that the requirements presented in this document are
not limited to the requirements of packet switched networks, but
also encompass the requirements of TDM, lambda and port switching
networks managed by Generalized MPLS (GMPLS) protocols. Protocol
solutions developed to meet the requirements set out in this
document must attempt to be equally applicable to MPLS and GMPLS.
Existing MPLS TE mechanisms such as [RFC3209] do not support P2MP TE
LSPs so new mechanisms must be developed. This should be achieved
with maximum re-use of existing MPLS protocols.
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Note that there is a separation between routing and signaling in
MPLS TE. In particular, the path of the MPLS TE LSP is determined by
performing a constraint-based computation (such as CSPF) on a traffic
engineering database (TED). The contents of the TED may be collected
through a variety of mechanism - extensions to the IGPs are a
popular mechanism for P2P MPLS TE.
This document focuses on requirements for establishing and
maintaining P2MP MPLS TE LSPs through signaling protocols; and
routing protocols are out of scope. No assumptions are made about how
the TED used as the basis for path computations for P2MP LSPs is
formed.
A P2MP TE LSP will be set up with TE constraints and will allow
efficient packet or data replication at various branching points in
the network. Note that the notion of "efficient" packet replication
is relative and may have different meanings depending on the
objectives (see section 4.2).
P2MP TE LSP setup mechanisms MUST include the ability to add/remove
receivers to/from an existing P2MP TE LSP.
1.1 Non-Objectives
For clarity, this section lists some items that are out of scope of
this document.
It is assumed that some information elements describing the P2MP TE
LSP are known to the ingress LSR prior to LSP establishment.
For example, the ingress LSRs knows the IP addresses that identify
the egress LSRs of the P2MP TE LSP. The mechanisms by which the
ingress LSR obtains this information is outside the scope of P2MP TE
signaling and so is not included in this document. Other documents
may complete the description of this function by providing
automated, protocol-based ways of passing this information to the
ingress LSR.
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- Non-TE LSPs (such as per-hop, routing-based LSPs).
- Discovery of egress leaves for a P2MP LSP
- Hierarchical P2MP LSPs
- OAM for P2MP LSPs
- Inter-area and inter-AS P2MP TE LSPs
- Applicability of P2MP MPLS TE LSPs to service scenarios
- Specific application or application requirements
- Algorithms for computing P2MP distribution trees
- Multipoint-to-point LSPs
- Multipoint-to-multipoint LSPs
- Routing protocols
- Construction of the traffic engineering database
- Distirbution of the information used to construct the traffic
engineering database
2. Definitions2.1 Acronyms
P2P:
Point-to-point
P2MP:
Point-to-multipoint
2.2 Terminology
The reader is assumed to be familiar with the terminology in
[RFC3031] and [RFC3209].
P2MP TE LSP:
A traffic engineered label switched path that has one unique
ingress LSR (also referred to as the root) and one or more
egress LSRs (also referred to as the leaf).
P2MP tree:
The ordered set of LSRs and links that comprise the path of a
P2MP TE LSP from its ingress LSR to all of its egress LSRs.
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ingress LSR:
The LSR that is responsible for initiating the signaling
messages that set up the P2MP TE LSP.
egress LSR:
One of potentially many destinations of the P2MP TE LSP.
Egress LSRs may also be referred to as leaf nodes or leaves.
bud LSR:
An LSR that is an egress, but also has one or more directly
connected downstream LSRs.
branch LSR:
An LSR that has more than one directly connected downstream LSR.
graft LSR:
An LSR that is already a member of the P2MP tree and is in
process of signaling a new sub-P2MP tree.
prune LSR:
An LSR that is a member of the P2MP tree and is in
process of tearing down an existing sub-P2MP tree.
P2MP-ID (Pid):
A unique identifier of a P2MP TE LSP, that is constant for the
whole LSP regardless of the number of branches and/or leaves.
2.2.1 Terminology for Partial LSPs
It is convenient to sub-divide P2MP trees for functional and
representational reasons. A tree may be divided in two dimensions:
- A division may be made along the length of the tree. For example,
the tree may be split into two components each running from the
ingress LSR to a discrete set of egress LSRs
- A tree may be divided at a branch LSR (or any transit LSR) to
produce a component of the tree that runs from the branch (or
transit) LSR to all downstream egress LSRs.
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These two methods of splitting the P2MP tree can be combined, so it
is useful to introduce some terminology to allow the partitioned
trees to be clearly described.
Use the following designations:
Source (ingress) LSR - S
Leaf (egress) LSR - L
Branch LSR - B
Transit LSR - X
Define three terms:
Sub-LSP
A component of the P2MP LSP that runs from one LSR to another
without (or ignoring) any branches.
Sub-tree
A component of the P2MP LSP that runs from one LSR to more than
one other LSR by branching.
Tree
A component of the P2MP LSP that runs from one LSR to all
downstream LSRs.
Using these new concepts we can define any combination or split of
the P2MP tree. For example:
S2L sub-LSP
The path from the source to one specific leaf.
S2L sub-tree
The path from the source to a set of leaves.
B2L tree
The path from a branch LSR to all downstream leaves.
X2X sub-LSP
A component of the P2MP LSP that is a simple path with
no branches.
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Internet Draft draft-ietf-mpls-p2mp-sig-requirement-00.txt December 20042.3 Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in [RFC2119].
3. Problem Statement3.1 Motivation
As described in section 1, Traffic Engineering and Constraint Based
Routing, including Call Admission Control(CAC), explicit source
routing and bandwidth reservation, are required to enable efficient
resource usage and strict QoS guarantees. Such mechanisms also make
it possible to provide services across a congested network where
conventional "shortest path first" forwarding paradigms would fail.
Existing MPLS TE mechanisms [RFC3209] and GMPLS TE mechanisms
[RFC3473] only provide support for P2P TE LSPs. While it is possible
to provide P2MP TE services using P2P TE LSPs, any such approach is
potentially suboptimal since it may result in data replication at
the ingress LSR, or in duplicate data traffic within the network.
Hence, to provide P2MP MPLS TE services in a fully efficient manner
it is necessary to specify specific requirements. These requirements
can then be used to define mechanisms for the use of existing
protocols and/or extensions to existing protocols and/or new
protocols.
3.2. Requirements Overview
This document states basic requirements for the setup of P2MP TE
LSPs. The requirements apply to the signaling techniques only, and no
assumptions are made about which routing protocols are run within the
network, nor about how the information that is used to construct the
Traffic Engineering Database (TED) is distributed. These factors are
out of the scope of this document.
A P2MP TE LSP path will be computed taking into account various
constraints such as bandwidth, affinities, required level of
protection and so on. The solution MUST allow for the computation
of P2MP TE LSP paths satisfying constraints with the objective of
supporting various optimization criteria such as delays, bandwidth
consumption in the network, or any other combinations. This is likely
to require the presence of a TED, as well as the ability to signal
the explicit path of an LSP.
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A desired requirement is also to maximize the re-use of existing
MPLS TE techniques and protocols where doing so does not adversely
impact the function, simplicity or scalability of the solution.
This document does not restrict the choice of signaling protocol
used to set up a P2MP TE LSP, but it should be noted that [RFC3468]
states
... the consensus reached by the Multiprotocol Label Switching
(MPLS) Working Group within the IETF to focus its efforts on
"Resource Reservation Protocol (RSVP)-TE: Extensions to RSVP for
Label-Switched Paths (LSP) Tunnels" (RFC 3209) as the MPLS signaling
protocol for traffic engineering applications...
The P2MP TE LSP setup mechanism MUST include the ability to
add/remove egress LSRs to/from an existing P2MP TE LSP and MUST
allow for the support of all the TE LSP management procedures
already defined for P2P TE LSP such as the non disruptive rerouting
(the so called "Make before break" procedure).
The computation of P2MP TE trees is implementation dependent and is
beyond the scope of the solutions that are built with this document
as a guideline.
Consider the following figure.
Source 1 (S1)
|
I-LSR1
| |
| |
R2----E-LSR3--LSR1 LSR2---E-LSR2--Receiver 1 (R1)
| :
R3----E-LSR4 E-LSR5
| :
| :
R4 R5
Figure 1
Figure 1 shows a single ingress (I-LSR1), and four egresses(E-LSR2,
E-LSR3, E-LSR4 and E-LSR5). I-LSR1 is attached to a traffic source
that is generating traffic for a P2MP application.
Receivers R1, R2, R3 and R4 are attached to E-LSR2, E-LSR3 and
E-LSR4.
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The following are the objectives of P2MP LSP establishment and use.
a) A P2MP TE LSP tree which satisfies various constraints is
pre-determined and supplied to ingress I-LSR1.
Note that no assumption is made on whether the tree is
provided to I-LSR1 or computed by I-LSR1. Note that the
solution SHOULD also allow for the support of partial path by
means of loose routing.
Typical constraints are bandwidth requirements, resource class
affinities, fast rerouting, preemption, to mention a few of
them. There should not be any restriction on the possibility
to support the set of constraints already defined for point to
point TE LSPs. A new constraint may specify which LSRs should
be used as branch points for the P2MP LSR in order to take
into account some LSR capabilities or network constraints.
b) A P2MP TE LSP is set up from I-LSR1 to E-LSR2, E-LSR3 and
E-LSR4 using the tree information.
c) In this case, the branch LSR1 should replicate incoming
packets or data and send them to E-LSR3 and E-LSR4.
d) If a new receiver (R5) expresses an interest in receiving
traffic, a new tree is determined and a sub-P2MP tree from
LSR2 to E-LSR5 is grafted onto the P2MP tree. LSR2 becomes a
branch LSR.
4. Detailed requirements for P2MP TE extensions4.1 P2MP LSP tunnels
The P2MP TE extensions MUST be applicable to the signaling of LSPs
of different traffic types. For example, it MUST be possible to
signal a P2MP TE LSP to carry any kind of payload being packet or
non-packet based (including frame, cell, TDM un/structured, etc.)
As with P2P MPLS technology [RFC3031], traffic is classified with a
FEC in this extension. All packets which belong to a particular FEC
and which travel from a particular node MUST follow the same P2MP
tree.
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In order to scale to a large number of branches, P2MP TE LSPs SHOULD
be identified by a unique identifier (the P2MP ID or Pid) that is
constant for the whole LSP regardless of the number of branches
and/or leaves. Therefore, the identification of the P2MP session by
its destination addresses is not adequate.
4.2 P2MP explicit routing
Various optimizations in P2MP tree formation need to be applied to
meet various QoS requirements and operational constraints.
Some P2MP applications may request a bandwidth guaranteed P2MP tree
which satisfies end-to-end delay requirements. And some operators
may want to set up a cost minimum P2MP tree by specifying branch
LSRs explicitly.
The P2MP TE solution therefore MUST provide a means of establishing
arbitrary P2MP trees under the control of an external tree
computation process or path configuration process or dynamic tree
computation process located on the ingress LSR. Figure 4 shows two
typical examples.
A A
| / \
B B C
| / \ / \
C D E F G
| / \ / \/ \ / \
D--E*-F*-G*-H*-I*-J*-K*--L H I J KL M N O
Steiner P2MP tree SPF P2MP tree
Figure 4 Examples of P2MP TE LSP topology
One example is the Steiner P2MP tree (Cost minimum P2MP tree)
[STEINER]. This P2MP tree is suitable for constructing a cost
minimum P2MP tree so as to minimize the bandwidth consumption in
the core. To realize this P2MP tree, several intermediate LSRs must
be both MPLS data terminating LSRs and transit LSRs (LSRs E, F, G,
H, I, J and K in the figure 4). This means that the LSRs must perform
both label swapping and popping at the same time. Therefore, the P2MP
TE solution MUST support a mechanism that can setup this kind of bud
LSR between an ingress LSR and egress LSRs. Note that this includes
constrained Steiner trees that allow for the computation of a minimal
cost trees with some other constraints such as a bounded delay
between the source and every receiver.
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Another example is a CSPF (Constraint Shortest Path First) P2MP
tree. By some metric (which can be set upon any specific criteria
like the delay, bandwidth, a combination of those), one can
calculate a shortest path P2MP tree. This P2MP tree is suitable for
carrying real time traffic.
The solution MUST allow the operator to make use of any tree
computation technique. In the former case an efficient/optimal tree
is defined as a minimal cost tree (Steiner tree) whereas in the
later case it is defined as the tree that provides shortest path
between the source and any receiver.
To support explicit setup of any reasonable P2MP tree shape, a P2MP
TE solution MUST support some form of explicit source-based control
of the P2MP tree which can explicitly include particular LSRs as
branch nodes. This can be used by the ingress LSR to setup the P2MP
TE LSP. For instance, a P2MP TE LSP can be simply represented as a
whole tree or by its individual branches.
4.3 Explicit Path Loose Hops and Widely Scoped Abstract Nodes
A P2MP tree is completely specified if all of the required branches
and hops between a sender and leaf LSR are indicated.
A P2MP tree is partially specified if only a subset of intermediate
branches and hops are indicated. This may be achieved using loose
hops in the explicit path, or using widely scoped abstract nodes
such as IPv4 prefixes shorter than 32 bits, or AS numbers.
A partially specified P2MP tree might be particularly useful in
inter-area and inter-AS situations although P2MP requirements for
inter-area and inter-AS are beyond the scope of this document.
Protocol solutions SHOULD include a way to specify loose hops and
widely scoped abstract nodes in the explicit source-based control
of the P2MP tree as defined in the previous section. Where this
support is provided, protocol solutions MUST allow downstream LSRs
to apply further explicit control to the P2MP tree to resolve a
partially specified tree into a (more) completely specified tree.
Protocol solutions MUST allow the P2MP tree to be completely
specified at the ingress where sufficient information exists to
allow the full tree to be computed.
In all cases, the egress nodes of the P2MP TE LSP must be fully
specified.
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In case of a tree being computed by some downstream LSRs (e.g. the
case of hops specified as loose hops), the solution MUST provide
the ability for the ingress LSR of the P2MP TE LSP to learn the full
P2MP tree. Note that this requirement MAY be relaxed in some
environments (e.g. Inter-AS) where confidentiality must be preserved.
4.4 P2MP TE LSP establishment, teardown, and modification mechanisms
The P2MP TE solution MUST support establishment, maintenance and
teardown of P2MP TE LSPs in a scalable manner. This MUST include
both the existence of very many LSPs at once, and the existence of
very many destinations for a single P2MP LSP.
In addition to P2MP TE LSP establishment and teardown mechanism, it
SHOULD implement partial P2MP tree modification mechanism.
For the purpose of adding sub-P2MP TE LSPs to an existing P2MP TE
LSP, the extensions SHOULD support a grafting mechanism. For the
purpose of deleting a sub-P2MP TE LSPs from an existing P2MP TE LSP,
the extensions SHOULD support a pruning mechanism.
It is RECOMMENDED that these grafting and pruning operations do not
cause any additional processing in nodes except along the path to
the grafting and pruning node and its downstream nodes. Moreover,
both grafting and pruning operations MUST not be traffic disruptive
for the traffic currently forwarded along the P2MP tree.
4.5 Fragmentation
The P2MP TE solution MUST handle the situation where a single
protocol message cannot contain all of the information necessary to
signal the establishment of the P2MP LSP. It MUST be possible to
establish the LSP in these circumstances.
This situation may arise in either of the following circumstances.
a. The ingress LSR cannot signal the whole tree in a single
message.
b. The information in a message expands to be too large (or is
discovered to be too large) at some transit node. This may
occur because of some increase in the information that needs
to be signaled or because of a reduction in the size of
signaling message that is supported.
The solution to these problems SHOULD NOT rely on IP fragmentation,
it is RECOMMENDED to rely on some protocol procedures specific to
the signaling solution.
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It is NOT RECOMMENDED that fragmented protocol messages are
re-combined at any downstream LSR.
4.6 Failure Reporting and Error Recovery
Failure events may cause egress nodes or sub-P2MP LSPs to become
detached from the P2MP TE LSP. These events MUST be reported
upstream as for a P2P LSP.
The solution SHOULD provide recovery techniques such as protection
and restoration allowing recovery of any impacted sub-P2MP TE
LSPs. In particular, a solution MUST provide fast protection
mechanisms applicable to P2MP TE LSP similar to the solutions
specified in [FRR] for P2P TE LSPs. Note also that no assumption is
made on whether backup paths for P2MP TE LSPs should or should not
be shared with P2P TE LSPs backup paths.
Note that the functions specified in [FRR] are currently specific to
packet environments and do not apply to non-packet environments.
Thus, while solutions MUST provide fast protection mechanisms
similar to those specified in [FRR], this requirement is limited to
the subset of the solution space that applies to packet switched
networks only.
Note that application-specific requirement documents may introduce
even more stringent requirement, such as no packet loss, as a
trade-off for the relaxation of other requirements, such as increased
bandwidth consumption.
The solution SHOULD also support the ability to meet other network
recovery requirements such as bandwidth protection and bounded
propagation delay increase along the backup path during failure.
A P2MP TE solution MUST support P2MP fast protection mechanism to
handle P2MP applications sensitive to traffic disruption.
The report of the failure of delivery to fewer than all of the
egress nodes SHOULD NOT cause automatic teardown of the P2MP TE LSP.
That is, while some egress nodes remain connected to the P2MP tree
it should be a matter of local policy at the ingress whether the
P2MP LSP is retained.
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When all egress nodes downstream of a branch node have become
disconnected from the P2MP tree, and the some branch node is unable
to restore connectivity to any of them by means of some recovery or
protection mechanisms, the branch node MAY remove itself from the
P2MP tree provided that it is not also an egress LSR. Since the
faults that severed the various downstream egress nodes from the
P2MP tree may be disparate, the branch node MUST report all such
errors to its upstream neighbor. The ingress node can then decide
to re-compute the path to those particular egress nodes, around the
failure point.
Solutions MAY include the facility for transit LSRs and particularly
branch nodes to recompute sub-P2MP trees to restore them after
failures. In the event of successful repair, error notifications
SHOULD NOT be reported to upstream nodes, but the new paths are
reported if route recording is in use. Crankback requirements are
discussed in Section 4.23.
4.7 Record route of P2MP TE LSP tunnels
Being able to identify the established topology of P2MP TE LSP is
very important for various purposes such as management and operation
of some local recovery mechanisms like Fast Reroute [FRR]. A network
operator uses this information to manage P2MP TE LSPs. Therefore,
topology information MUST be collected and updated after P2MP TE LSP
establishment and modification process.
The P2MP TE solution MUST support a mechanism which can collect and
update P2MP tree topology information after P2MP LSP establishment
and modification process. For example, the P2P MPLS TE mechanism of
route recording could be extended and used if RSVP-TE was used as
the P2MP signaling protocol.
It is RECOMMENDED that the information is collected in a data format
by which the sender node can recognize the P2MP tree topology
without involving some complicated data calculation process.
The solution MUST support the recording of both outgoing interfaces
and node-id.
4.8 Call Admission Control (CAC) and QoS Control mechanism of P2MP TE LSPs
P2MP TE LSPs may share network resource with P2P TE LSPs. Therefore
it is important to use CAC and QoS in the same way as P2P TE LSPs
for easy and scalable operation.
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P2MP TE solutions MUST support both resource sharing and exclusive
resource utilization to facilitate co-existence with other LSPs to
the same destination(s).
P2MP TE solution MUST be applicable to DiffServ-enabled networks
that can provide consistent QoS control in P2MP LSP traffic.
Any solution SHOULD also satisfy the DS-TE requirements [RFC3564]
and interoperate smoothly with current P2P DS-TE protocol
specifications.
Note that this requirement document does not make any assumption on
the type of bandwidth pool used for P2MP TE LSPs which can either be
shared with P2P TE LSP or be dedicated for P2MP use.
4.9 Variation of LSP Parameters
Various parameters to an LSP (such as priority, bandwidth, etc.) are
signaled along each branch of the LSP.
Any solution MUST NOT allow for variance of these parameters. That
is,
- no attributes set and signaled by the ingress of a P2MP LSP may be
varied by downstream LSRs
- there MUST be homogeneous QoS from the root to all leaves.
THIS IS A PROVISIONAL REQUIREMENT STILL OPEN FOR DISCUSSION.
4.10 Re-optimization of P2MP TE LSPs
The detection of a more optimal path (for example, one with a lower
overall cost) is an example of a situation where P2MP TE LSP
re-routing may be required. While re-routing is in progress, an
important requirement is avoiding double bandwidth reservation
(over the common parts between the old and new LSP) thorough the use
of resource sharing.
Make-before-break MUST be supported for a P2MP TE LSP to ensure that
there is minimal traffic disruption when the P2MP TE LSP is
re-routed.
It is possible to achieve make-before-break that only applies to a
sub-P2MP tree without impacting the data on all of the other parts
of the P2MP tree.
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Internet Draft draft-ietf-mpls-p2mp-sig-requirement-00.txt December 2004
The solution SHOULD allow for make-before-break re-optimization of
any subdivision of the P2MP LSP (S2L sub-tree, S2X sub-LSP, S2L
sub-LSP, X2L sub-tree, B2L sub-tree, X2L tree, or B2L tree) with no
impact on the rest of the P2MP LSP (no label reallocation, no change
in identifiers, etc.).
The solution SHOULD also provide the ability for the ingress LSR to
have a strict control on the re-optimization process.
Such re-optimization MAY be initiated by the sub-tree root branch
node (that is, the branch node MAY setup a new sub-tree, then splice
traffic on the new subtree and delete the former sub-tree).
THE REQUIREMENT FOR RE-OPTIMIZATION BY SUB-TREE ROOT BRANCH IS
STILL OPEN FOR DISCUSSION
4.11 Tree Remerge
It is possible for a single transit LSR to receive multiple
signaling messages for the same P2MP LSP but for different
sets of destinations. These messages may be received from the
same or different upstream nodes and may need to be passed on
to the same or different downstream nodes.
This situation may arise as the result of the signaling solution
definition or implementation options within the signaling
solution. Further, it may happen during make-before-break
reoptimization (section 4.10), or as a result of signaling
message fragmentation (section 4.5).
It is even possible that it is necessary to construct distinct
upstream branches in order to achieve the correct label choices
in certain switching technologies managed by GMPLS (for example,
photonic cross-connects where the selection of a particular
lambda for the downstream branches is only available on different
upstream switches).
The solution MUST handle the case where multiple signaling
messages for the same P2MP LSP are received at a single transit
LSR with the end result of all receivers being added to the
P2MP LSP.
THIS REQUIREMENT IS STILL UNDER DISCUSSION
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Internet Draft draft-ietf-mpls-p2mp-sig-requirement-00.txt December 20044.12 Data Duplication
Data duplication refers to the receipt by any recipient of duplicate
instances of the data. In a packet environment this means the
receipt of duplicate packets - although this should be a benign (if
inefficient) situation, it may be catastrophic in certain existing
and deployed applications. In a non-packet environment this means
the duplication in time of some part of the signal that may lead to
the replication of data or to the scrambling of data.
Data duplication may legitimately arise in various scenarios
including re-optimization of active LSPs as described in the
previous section, and protection of LSPs. Thus, it is impractical to
regulate against data duplication in this document.
Instead, the solution MUST provide a mechanism to resolve, limit or
avoid data duplication at either or both of:
- the point at which the data path diverges
- the point at which the data paths converge.
THE EXTENT TO WHICH DATA DUPLICATION MAY BE TOLERATED (in time or in
a count of bits or packets) IS FOR FURTHER STUDY.
4.13 IPv4/IPv6 support
Any P2MP TE solution MUST be equally applicable to IPv4 and IPv6.
4.14 P2MP MPLS Label
A P2MP TE solution MUST support establishment of both P2P and P2MP
TE LSPs and MUST NOT impede the operation of P2P TE LSPs within the
same network. A P2MP TE solution MUST be specified in such a way
that it allows P2MP and P2P TE LSPs to be signaled on the same
interface. Labels for P2MP TE LSPs and P2P TE LSPs MAY be assigned
from shared or dedicated label space(s). Label space shareability is
implementation specific.
4.15 Routing advertisement of P2MP capability
Several high-level requirements have been identified to determine
the capabilities of LSRs within a P2MP network. The aim of such
information is to facilitate the computation of P2MP trees using TE
constraints within a network that contains LSRs that do not all have
the same capabilities levels with respect to P2MP signaling and data
forwarding.
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Internet Draft draft-ietf-mpls-p2mp-sig-requirement-00.txt December 2004
These capabilities include, but are not limited to:
- the ability of an LSR to support branching.
- the ability of an LSR to act as an egress and a branch for the
same LSP.
- the ability of an LSR to support P2MP MPLS-TE signaling.
It is expected that it may be appropriate to gather this information
through extensions to TE IGPs (see [RFC3630] and [IS-IS-TE]), but
the precise requirements and mechanisms are out of the scope of this
document. It is expected that a separate document will cover this
requirement.
4.16 Multi-Area/AS LSP
P2MP TE solutions SHOULD support multi-area/AS P2MP TE LSPs.
The precise requirements in support of multi-area/AS P2MP TE LSPs is
out of the scope of this document. It is expected that a separate
document will cover this requirement.
4.17 Multi-access LANs
P2MP MPLS TE may be used to traverse network segments that are
provided by multi-access media such as Ethernet. In these cases, it
is also possible that the entry point to the network segment is a
branch point of the P2MP LSP.
Two options clearly exist:
- the branch point replicates the data and transmits multiple
copies onto the segment
- the branch point sends a single copy of the data to the segment
and relies on the exit points to discriminate the reception of
the data.
The first option has a significant scaling issue since all
replicated data must be sent through the same port and carried on
the same segment. Thus, a solution SHOULD provide a mechanism for a
branch node to send a single copy of the data onto a multi-access
network and reach multiple (adjacent) downstream nodes.
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Internet Draft draft-ietf-mpls-p2mp-sig-requirement-00.txt December 20044.18 P2MP MPLS OAM
Management of P2MP LSPs is as important as the management of P2P
LSPs.
The MPLS and GMPLS MIB modules MUST be enhanced to provide P2MP TE
LSP management.
In order to facilitate correct management, P2MP TE LSPs MUST have
unique identifiers.
OAM facilities will have special demands in P2MP environments
especially within the context of tracing the paths and connectivity
of P2MP TE LSPs. The precise requirements and mechanisms for OAM are
out of the scope of this document. It is expected that a separate
document will cover these requirements.
4.19 Scalability
Scalability is a key requirement in P2MP MPLS systems. Solutions
MUST be designed to scale well with an increase in the number of any
of the following:
- the number of recipients
- the number of branch points
- the number of branches.
Both scalability of performance and operation MUST be considered.
Key considerations SHOULD include:
- the amount of refresh processing associated with maintaining
a P2MP TE LSP.
- the amount of protocol state that must be maintained by ingress
and transit LSRs along a P2MP tree.
- the number of protocol messages required to set up or tear down a
P2MP LSP as a function of the number of egress LSRs.
- the number of protocol messages required to repair a P2MP LSP
after failure or perform make-before-break.
- the amount of protocol information transmitted to manage
a P2MP TE LSP (i.e. the message size).
- the amount of potential routing extensions.
- the amount of control plane processing required by the ingress,
transit and egress LSRs to add/delete a branch LSP to/from an
existing P2MP LSP.
It is expected that the applicability of each solution will be
evaluated with regards to the aforementioned scalability criteria.
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Internet Draft draft-ietf-mpls-p2mp-sig-requirement-00.txt December 20044.19.1 Absolute Limits
THIS IS SECTION DESCRIBES PROVISIONAL REQUIREMENTS STILL OPEN FOR
DISCUSSION.
In order to achieve the best solution for the problem space it is
helpful to clarify the boundaries for P2MP TE LSPs.
- Number of recipients.
A P2MP TE LSP MUST reduce to similar scaling properties as a P2P
LSP when the number of recipients reduces to one.
It is important to classify the problem as a Traffic Engineering
problem. It is anticipated that the initial deployments of P2MP TE
LSPs may be limited to only several hundred recipients, but also
that future deployments may require significantly larger numbers.
An acceptable solution, therefore, is one that scales linearly
with the number of recipients.
Solutions that scale worse than linear (that is, exponential or
polynomial) are not acceptable whatever the number of recipients
they could support
- Number of branch points.
Solutions MUST support all possibilities from one extreme of a
single branch point that forks to all leaves on a separate branch,
to the greatest number of branch points which is (n-1) for n
recipients. Assumptions MUST NOT be made in the solution regarding
which topology is more common, and the solution MUST be designed
to ensure scalability in all topologies.
- Dynamics of P2MP tree.
Recall that the mechanisms for determining which recipients should
be added to an LSP, and for adding and removing recipients from
that group are out of the scope of this document. Nevertheless, it
is useful to understand the expected rates of arrival and
departure of recipients since this can impact the selection of
solution techniques.
Again, it must be recalled that this document is limited to Traffic
Engineering, and in this model the rate of change of recipients
may be expected to be lower than in an IP multicast group.
Although the absolute number of recipients coming and going is the
important element for determining the scalability of a solution,
it may be noted that a percentage may be a more comprehensible
measure but that this is not as significant for LSPs with a small
number of recipients.
A working figure for an established P2MP TE LSP is less than 10%
churn per day. That is, a relatively slow rate of churn.
We could say that a P2MP LSP would be shared by multiple multicast
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Internet Draft draft-ietf-mpls-p2mp-sig-requirement-00.txt December 2004
groups and dynamics of P2MP LSP would be relatively small.
Considering applicability that P2MP LSP to use L2 multi-access
path technology, we can consider stable P2MP L2 path even when we
transfer IP multicast traffic over the path.
Solutions MUST optimize around such relatively low rates of change
and are NOT REQUIRED to optimize for significantly higher rates
of change.
- Rate of change within the network.
It is also important to understand the scaling with regard to
changes within the network. That is, one of the features of a
P2MP TE LSP is that it can be robust or protected against network
failures, and can be re-optimized to take advantage of newly
available network resources.
It is more important that a solution be optimized for scaling with
respect to recovery and re-optimization of the LSP, than for change
in the recipients, because P2MP is used as a TE tool.
The solution MUST follow this distinction.
4.20 Backwards Compatibility
It SHOULD be an aim of any P2MP solution to offer as much backward
compatibility as possible. An ideal which is probably impossible to
achieve would be to offer P2MP services across legacy MPLS networks
without any change to any LSR in the network.
If this ideal cannot be achieved, the aim SHOULD be to use legacy
nodes as both transit non-branch LSRs and egress LSRs.
It is a further requirement for the solution that any LSR that
implements the solution SHALL NOT be prohibited by that act from
supporting P2P TE LSPs using existing signaling mechanisms. That is,
unless administratively prohibited, P2P TE LSPs MUST be supported
through a P2MP network.
Also, it is a requirement that P2MP TE LSPs MUST be able to co-exist
with IP unicast and IP multicast networks.
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Internet Draft draft-ietf-mpls-p2mp-sig-requirement-00.txt December 20044.21 GMPLS
The requirement for P2MP services for non-packet switch interfaces
is similar to that for PSC interfaces. Therefore, it is a requirement
that reasonable attempts must be made to make all the features/
mechanisms (and protocol extensions) that will be defined to provide
MPLS P2MP TE LSPs equally applicable to P2MP PSC and non-PSC TE-LSPs.
If the requirements of non-PSC networks over-complicate the PSC
solution a decision may be taken to separate the solutions. This
decision must be taken in full consultation with the MPLS and CCAMP
working groups.
Solutions for MPLS P2MP TE-LSPs when applied to GMPLS P2MP PSC or
non-PSC TE-LSPs MUST be backward and forward compatible with the
other features of GMPLS including:
- control and data plane separation (IF_ID RSVP_HOP and IF_ID
ERROR_SPEC),
- full support of numbered and unnumbered TE links (see [RFC 3477]
and [GMPLS-ROUTE]),
- use of the GENERALIZED_LABEL_REQUEST, the GENERALIZED_LABEL
(C-Type 2 and 3), the SUGGESTED_LABEL and the RECOVERY_LABEL,
in conjunction with the LABEL_SET and the ACCEPTABLE_LABEL_SET
object,
- processing of the ADMIN_STATUS object,
- processing of the PROTECTION object,
- support of Explicit Label Control,
- processing of the Path_State_Removed Flag,
- handling of Graceful Deletion procedures.
- E2E and Segment Recovery procedures.
- support of Graceful Restart
In addition, since non-PSC TE-LSPs may have to be processed in
environments where the "P2MP capability" could be limited, specific
constraints may also apply during the P2MP TE Path computation.
Being technology specific, these constraints are outside the scope
of this document. However, technology independent constraints
(i.e. constraints that are applicable independently of the LSP
class) SHOULD be allowed during P2MP TE LSP message processing.
It has to be emphasized that path computation and management
techniques shall be as close as possible to those being used for
PSC P2P TE LSPs and P2MP TE LSPs.
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Internet Draft draft-ietf-mpls-p2mp-sig-requirement-00.txt December 20044.22 Requirements for Hierarchical P2MP TE LSPs
[LSP-HIER] defines concepts and procedures for P2P LSP hierarchy.
The P2MP MPLS-TE solution SHOULD support the concept of region and
region hierarchy (PSC1<PSC2<PSC3<PSC4<L2SC<TDM<LSC<FSC).
Particularly it SHOULD allow a Region i P2MP TE LSP to be nested
into a region j P2MP TE LSP or multiple region j P2P TE LSPs,
providing that i<j.
The precise requirements and mechanisms for this function are out of
the scope of this document. It is expected that a separate document
will cover these requirements.
4.23 P2MP Crankback routing
P2MP solutions SHOULD support crankback requirements as defined in
[CRANKBACK]. In particular, they SHOULD provide sufficient
information to a branch LSR from downstream LSRs to allow the branch
LSR to re-route a sub-tree around any failures or problems in the
network.
5. Security Considerations
This requirements document does not define any protocol extensions
and does not, therefore, make any changes to any security models.
It should be noted that P2MP signaling mechanisms built on P2P
RSVP-TE signaling are likely to inherit all of the security
techniques and problems associated with RSVP-TE. These problems may
be exacerbated in P2MP situations where security relationships may
need to maintained between an ingress and multiple egresses. Such
issues are similar to security issues for IP multicast.
It is a requirement that documents offering solutions for P2MP LSPs
MUST have detailed security sections.
6. Acknowledgements
The authors would like to thank George Swallow, Ichiro Inoue, Dean
Cheng, Lou Berger and Eric Rosen for their review and suggestions.
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Internet Draft draft-ietf-mpls-p2mp-sig-requirement-00.txt December 2004
Adrian Farrel
Old Dog Consulting
Phone: +44 (0) 1978 860944
Email: adrian@olddog.co.uk
Markus Jork
Avici Systems
101 Billerica Avenue
N. Billerica, MA 01862
Phone: +1 978 964 2142
Email: mjork@avici.com
Andrew G. Malis
Tellabs
2730 Orchard Parkway
San Jose, CA 95134
Phone: +1 408 383 7223
Email: andy.malis@tellabs.com
Jean-Louis Le Roux
France Telecom
2, avenue Pierre-Marzin
22307 Lannion Cedex
France
Email: jeanlouis.leroux@francetelecom.com
10. Intellectual Property Consideration
The IETF takes no position regarding the validity or scope of any
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such rights. Information on the procedures with respect to rights
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Copies of IPR disclosures made to the IETF Secretariat and any
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specification can be obtained from the IETF on-line IPR repository
at http://www.ietf.org/ipr.
Yasukawa, et. al. [Page 29]

Internet Draft draft-ietf-mpls-p2mp-sig-requirement-00.txt December 2004
The IETF invites any interested party to bring to its attention
any copyrights, patents or patent applications, or other
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to implement this standard. Please address the information to the
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11. Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is
subject to the rights, licenses and restrictions contained in BCP78, and except as set forth therein, the authors retain all their
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This document and the information contained herein are provided
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Yasukawa, et. al. [Page 30]